python

<!-- Last reviewed: 2026-03 --> # Python Domain Skill ## Modern Type Hints (3.10+) ```python # Union with | operator def process(value: int | str | None) -> dict[str, int]: ... # Type aliases type UserID = int type Config = dict[str, str | int | bool] # See references/type-hints-advanced.md for: # - Generics and TypeVars # - Protocols (structural typing) # - Function overloading ``` ## Testing with Pytest ```python # Fixtures @pytest.fixture def sample_user(): return {"name": "Alice", "email": "alice@example.com"} # Parametrize @pytest.mark.parametrize("input,expected", [ (0, 0), (1, 1), (2, 4), ]) def test_square(input, expected): assert input ** 2 == expected # Mock with pytest-mock def test_api(mocker): mock_get = mocker.patch("requests.get") mock_get.return_value.json.return_value = {"status": "ok"} result = fetch_data() assert result["status"] == "ok" ``` **Coverage target: 80%+** ```bash pytest --cov=myapp --cov-report=term-missing --cov-fail-under=80 ``` **See references/testing-pytest.md for** advanced fixtures, mocking patterns, markers, plugins. ## Tooling ### Ruff (Linting + Formatting) Replaces Black, isort, flake8, pyupgrade. ```bash ruff check --fix . && ruff format . ``` ```toml # pyproject.toml [tool.ruff] line-length = 88 target-version = "py310" [tool.ruff.lint] select = ["E", "W", "F", "I", "N", "UP", "B", "C4", "SIM"] ignore = ["E501"] [tool.ruff.format] quote-style = "double" ``` ### Mypy (Type Checking) ```toml [tool.mypy] python_version = "3.10" strict = true warn_return_any = true disallow_untyped_defs = true ``` ### Poetry (Dependency Management) ```bash poetry init poetry add requests poetry add --group dev pytest pytest-cov ruff mypy poetry install poetry run pytest ``` **See references/tooling-config.md for** complete ruff rule sets, mypy options, pre-commit hooks, CI/CD. ## Async Programming ```python # Concurrent execution async def fetch_all(urls: list[str]) -> list[dict]: return await asyncio.gather(*[fetch_data(url) for url in urls]) # Async context manager class AsyncDatabase: async def __aenter__(self): self.connection = await connect() return self async def __aexit__(self, *args): await self.connection.close() ``` **See references/async-patterns.md for** task management, semaphores, queues, error handling, pitfalls. ## Data Modeling | Use case | Choose | Why | |----------|--------|-----| | Internal data, simple containers | `@dataclass` | Stdlib, fast, no deps | | Performance-critical, immutable | `@dataclass(frozen=True, slots=True)` | Minimal overhead | | External/untrusted data, APIs | `pydantic.BaseModel` | Auto-validation, serialization | | Config from environment | `pydantic_settings.BaseSettings` | Env var parsing, type coercion | ```python # Dataclass: internal data @dataclass(frozen=True) class Point: x: float y: float # Pydantic: external data with validation class UserCreate(BaseModel): name: str = Field(..., min_length=1, max_length=100) email: EmailStr age: int = Field(..., ge=0, le=150) @field_validator("email") @classmethod def normalize_email(cls, v: str) -> str: return v.lower().strip() ``` **See references/dataclasses-pydantic.md for** advanced features, nested models, settings, JSON schema. ## Anti-patterns Quick Reference | Anti-pattern | Fix | |-------------|-----| | Mutable default `def f(x=[])` | `def f(x=None)` then `x = x or []` | | Bare `except:` | `except SpecificError as e:` | | `== None` / `== True` | `is None` / `is True` | | Modify list while iterating | List comprehension filter | | `f = open()` without `with` | `with open() as f:` | | String concat in loop `+=` | `"".join(...)` | | Manual index counter | `enumerate()` | | `type(x) == list` | `isinstance(x, list)` | | `print()` debugging | `logging.getLogger(__name__)` | | Star imports `from x import *` | Explicit imports | ## Project Setup Checklist ```bash # 1. Initialize poetry init poetry add <dependencies> poetry add --group dev pytest pytest-cov ruff mypy pre-commit # 2. Configure pyproject.toml # [tool.ruff], [tool.mypy], [tool.pytest.ini_options] # 3. Install hooks pre-commit install # 4. Full quality check ruff check --fix . && ruff format . && mypy . && pytest --cov=myapp --cov-fail-under=80 ``` ## Commands ```bash ruff format . && ruff check --fix . # Format + lint mypy . # Type check pytest --cov=myapp --cov-report=term-missing --cov-fail-under=80 # Test + coverage ``` ## Reference Files - **references/type-hints-advanced.md** - Generics, Protocols, TypeVars, overloads, TypedDict - **references/async-patterns.md** - Task management, queues, synchronization, performance - **references/testing-pytest.md** - Fixtures, parametrize, mocking, markers, plugins - **references/tooling-config.md** - ruff, mypy, poetry, pre-commit, bandit configs - **references/dataclasses-pydantic.md** - Data modeling, validation, serialization, settings